Refrigeration system lubrication



Uite

3,375,197 Patented Mar. 26, 1968 ice The present invention relates tothe lubrication of refrigeration systems of the closed hermeticallysealed compression type including dichlorodifiuorom'ethane as arefrigerant and a mineral oil as a lubricant and is particularlyconcerned with such a system including an additive for the lubricantwhich is effective to inhibit reactions between the refrigerant and theoil.

In hermetically sealed refrigeration systems, in which the oil andrefrigerant are in direct heat exchange relationship with each other,the maximum operating temperature of the refrigeration compressor isoften limited by the chemical instability of the oil-refrigerantmixture. A refrigerant commonly used in hermetically sealed systems isdichlorodifluoromethane (R12) while various petroleum oils are employedas lubricants. When tested independently, both the refrigerant and theoil are stable for the expected operating life of the equipment attemperatures well above those which are normally attained in actualoperation. However, it has been known for some years that when mixed therefrigerant R12 and petroleum oils react at moderately elevatedtemperatures and that the reaction may result in the formation of carbonor coke deposits. It is also known that the rate of reaction isaccelerated in the presence of iron or steel and that steel valve platesare particularly susceptible to coking at elevated temperatures. Sincethe refrigerant R12 is soluble in petroleum oils and since the solutioncomes in direct contact with various components of the compressor whichare made from steel, it is highly desirable to provide means forpreventing or reducing the R12-mineral oil reaction. Various attempts toimprove the stability of the R12-oil working fluids at elevatedtemperatures have included the use of highly refined mineral oils freeof activating impurities which might accelerate or initiate the R12-oilreaction, and the use of various additives of the type found to beuseful in improving the stability or lubricity of lubricants in fieldsoutside the refrigeration field. However, such additives have not beenfound to be particularly effective in inhibiting or preventing the R12-oil reaction. In fact, it has been found that some well known oiladditives when added to a highly refined mineral oil lubricant mayactually lower the critical reaction temperature at which the R12-oilreaction takes place.

The present invention is based on the discovery that a certain class ofadditives previously used as lubricity additives for lubricating oils inother lubricating applications such as automobile engines but which havenot heretofore been considered or contemplated as possible solutions tothe R12-oil reaction in refrigeration systems are,

in fact, particularly effective in providing a R12-oil work- 5 ing fluidfor refrigerating systems having high temperatures.

More specifically, in accordance with the present invention there isincorporated into the oil employed to lubricate the refrigerationcompressor, a refrigerant-oil reaction inhibiting amounts of ametallodithiophosphate compound of the formula in which M represents adivalent metal, preferably zinc, cadmium or nickel and R represents thesame or different alkyl, aryl, alkaryl or aralkyl radicals containing atleast 3 carbon atoms and preferably radicals containing 6 or more carbonatoms. In general, it has been found that the improved results describedherein can be obtained when the additive concentration is from 0.01 to 3percent, preferably 0.02 to 1 percent, based on the weight of the oil.

As stated hereinbefore, it has been previously proposed to addmetallodithiophosphates to various lubricating compositions for thepurpose of improving their heat stability oxidation resistance andlubricity properties and it has been thought that the thermaldecomposition of such additives at elevated temperatures results in anin situ deposition of solid lubricating layers on the lubricatedsurfaces. Chemical studies of the effect of the zinc dithiophosphates onthe R12-oil reaction have now indicated that such additives apparentlyreact with iron and steel surfaces and in effect produce a coatingthereon which inhibits the catalytic or accelerating effect of suchsurfaces on the R12-oil reaction.

In order to demonstrate the effectiveness of suchmetallodithiophosphates in inhibiting the R12-oil reaction, sealed tubeswere prepared containing compressor valve steel, R12 refrigerant andmineral oil plus various amounts of various additives. The sealed tubeswere heated for various times at 175 C. and the contents of the tubeswere then analyzed for R12 decomposition which is known (H. O. Spauschusand G. C. Doderer, ASHRAE Journal, 3, No. 2. 65, 1961) to involve theformation of monochlorodifluoromethane (R22) in accordance with thefollowing type of reaction:

fied R12, 1 ml. of the specified refrigeration lubricant oil, with orwithout additive, and 0.05 gram of valve steel.

Percent Additive and R Time, lemp., Percent Test No Oil in R-Zn P DaysColor R12 Reacted Z-29 P 14 No Change 0.06 +1% TCP 14 175 SlightChange.-. 1.1

P 14 175 No Ohange 0. 003 14 175 do 0.002 14 175 .do .01

14 175 Black 1.2 14 175 No Change. .01 butyL 14 175 ..do 0.002 0.1%Propyl 14 175 ..do O. 002 4A1 N2 14 175 Yellow 0. 15

N2... 0.1% Octyl 14 175 No Change 0.008 N2. 0.3% Octyl 14 175 do 0.007N2. 0.1% Substituted Pheny1 14 175 do 0. 004 N2 8 175 Dark Brown 1. 634-5 N2 0.1% Octyl 175 No Change. 0.011

In the above tests, oil P was a highly refined waterwhite mineral oil ofthe parafinic type with a viscosity of 100 Saybolt Universal seconds at100 F. This oil is not suitableas a compressor lubricant withoutaddition of extreme pressure agents such as tricresylphosphate (TCP).Oil N1 was a moderately refined mapthenic pale oil from Gulf Coastcrudes, de-waxed for low temperature refrigeration use and with aviscosity of 200 Saybolt Universal seconds at 100 F. Oil N2 was a mediumrefined napthenic oil recovered from Gulf Coast crudes and with aviscosity of 150 Saybolt Universal seconds at 100 F. The substitutedphenyl dithiophosphate was a proprietary Zinc dithiophosphate containinghydrocarbon substituted phenyl groups.

From a review of these test results, it will be seen that the R12-oilreaction is effectively inhibited by the presence of relatively smalladditions of the metal dithiophosphates. Somewhat larger amounts can, ofcourse, also be used to effectively prevent or inhibit the formation ofcoke deposits on various portions of the compressor and particularly thevalve and valve areas.

Accelerated life tests on refrigerating systems containing various Zincdithiophosphate additives have indicated that the additive is consumedin a fairly short time under accelerated life test conditions. Forexample, an oil sample removed after thirty days of testing at 120 C.motor winding temperature contained only about of the initial additiveconcentration, but even at this low residual additive level reactionwith the R12-oil was inhibited. In units operated at 140160 C., zincanalysis has indicated that no additive remained after thirty daysalthough the R12-oil reaction was still at a substantially lower levelthan when no additive was present.,A1th0ugh the unit tests haveindicated that the additive is consumed at elevated temperatures,results have been shown that in all cases a permanent improvement hasbeen effected insofar as coke formation is concerned. In other words, itappears that the additive, while consumed, reacts with the steelsurfaces thereby altering their chemical activity.

In systems in which the compressor may operate at relatively hightemperatures, it has been found that best results are obtained when theR group contains at least 6 carbon atoms as for example when R is anaryl or a higher alkyl radical. Accelerated life tests on compressorsemploying different additives have indicated that dithiophosphatescontaining lower alkyl radicals, as for example the isobutyl and propylradicals, may produce varnish-like deposit on the valves and valveplates resulting from the thermal decomposition of the metallodithiophosphate additive. However, additives in which the R groupcontains at least 6 carbon atoms and particularly higher alkyl radicals,such as represented by zinc dioctyldithiophosphate, do not produce anyvarnish-like deposits on the valve and valve plate surfaces. Forexample, after six months operation in a compressor operat-. ing underload conditions producing a temperature of 180- 200 F. in the dischargeline, one inch from the case, an oil including .1% zincdioctyldithiophosphate as an additive exhibited no deposits on thevalves and no change in its operating temperatures. In addition, theused lubricant was found to contain about /3 of the initial additive anda sample of the lubricant subjected to the same sealed tube testemployed in obtaining the results set forth in above table after heatingfor 14 days at 175 C. indicated only a 0.001% reaction of the R12.

Extensive high temperature coking tests on a number of differenthermetic compressor have shown that zinc dithiophosphate additives willextend the compressor life at least three fold as compared with oils notcontaining such additives.

While the invention has been described with particularity in connectionwith certain specific metal dithiophosphates, it is obvious that it isnot limited thereto and that various modifications can be made withoutdeparting from the spirit and scope of the invention.

What we claim as new and desire to secure by Letters Patent of theUnited States is:

1. In the method of operating a refrigeration system of the hermeticallysealed, compression type employing dichlorodifluorornethane as arefrigerant and a mineral oil lubricant in contact with saidrefrigerant, the improvement which comprises lubricating saidrefrigeration system with a mineral oil lubricant having dissolvedtherein from 0.01 to 3 percent by weight of a compound of the formula inwhich M represents a metal selected from the group consisting of zinc,cadmium and nickel and R represents the same or different hydrocarbonradicals selected from the group consisting of alkyl, aryl, alkaryl andaralkyl radicals containing at least 3 carbon atoms, said compound beingeffective to inhibit reaction between said oil and said refrigerant inthe presence of steel.

2. In the method of operating a refrigeration system of the hermeticallysealed, compression type employing dichlorodifluoromethane as therefrigerant and a mineral oil lubricant in contact with saidrefrigerant, the improvement which comprises lubricating saidrefrigeration system with a mineral oil lubricant having dissolvedtherein from 0.01 to 3 percent by weight of a compound of the formula inwhich M represent a metal selected from the group consisting of zinc,cadmium and nickel and R represents a hydrocarbon radical selected fromthe group consisting of alkyl, aryl, alkaryl and aralkyl radicalscontaining at least 6 carbon atoms, said compound being effective toinhibit reaction between said oil and said refrigerant in the presenceof steel.

3. In the operation of a refrigeration system of the hermeticallysealed, compression type employing dichlorodifluoromethane as therefrigerant, and a mineral oil lubricant in contact with saidrefrigerant the improvement which comprises lubricating saidrefrigeration system with a mineral oil lubricant having dissolvedtherein from 0.01 to 3 percent by weight of a compound of the formula inwhich R represents an alkyl radical containing at least 3 carbon atoms,said compound being effective to inhibit reaction between said oil andsaid refrigerant in the presence of steel.

4. The operation of a refrigeration system as defined in claim 3 inwhich R is an octyl radical.

5. The operation of a refrigeration system as defined in claim 2 inwhich R is an alkaryl radical.

References Cited UNITED STATES PATENTS 2,734,865 2/1956 Peeso et al.252400 2,824,061 2/1958 Davidson et al. 62114 3,037,932 6/1962 Barker252l 5 X LEON D. ROSDOL, Primary Examiner.

J. D. WELSH, Assistant Examiner.

1. IN THE METHOD OF OPERATING A REFRIGERATION SYSTEM OF THE HERMETICALLYSEALED, COMPRESSION TYPE EMPLOYING DICHLORODIFLUOROMETHANE AS AREFRIGERANT AND A MINERAL OIL LUBRICANT IN CONTACT WITH SAIDREFRIGERANT, THE IMPROVEMENT WHICH COMPRISES LUBRICATING SAIDREFRIGERATION SYSTEM WITH A MINERAL OIL LUBRICANT HAVING DISSOLVEDTHEREIN FROM 0.01 TO 3 PERCENT BY WEIGHT OF A COMPOUND OF THE FORMULA